The present invention relates, in general, to check valves used in underground mining and, more particularly, to a new and useful check valve for use in connection with piston-cylinder arrangements in which the piston can be alternately subjected to hydraulic pressure in both sides such as are found in hydraulic props, thrust piston gears and pushing rams in underground mining.
In underground mining operations, the use of unlockable check valves is known in which such valves are used as recovery and setting valves for hydraulically operated mining props. The known unlockable check valves include a housing having a control space with a piston-type control element in which a piston rod is integrally connected on one side. The piston rod is displaceable in axially guided relationship with a housing extension and sealingly engages the housing extension. The piston rod projects toward the direction of a spring loaded shutoff device that can be forced into an open position by the movement of the piston rod against the restoring force of the spring. The shutoff device normally isolates a setting space in the cylinder housing in the hydraulic prop or the like, which is under fluid pressure, from a return pipe. The piston rod projects through an outlet space. The outlet space is connected to a return pipe which has a connection to the outlet side of the shutoff device. In addition, the outlet space is connected via conduits to the piston rod side of the control space, while the portion of the control space on the opposite side of the piston-type control element is connected to another return pipe which can be connected, however, to the same return flow as the return pipe connected to the outlet space. In this way, the piston-type control element is hydraulically pressure-equalized on both sides in the neutral position of the check valve, that is, after the respective prop or the like has been set. Accordingly, back pressure or back flow in the return flow channel cannot cause a displacement of the piston-type control element. Thus, the unlockable check valve maintains a neutral disposition, that is, accidental opening of the shutoff device is impossible. A disadvantage of this known design, however, is that the valve tends to flutter upon the occurrence of unavoidable expansion blows or pressure relief shocks.
Another known unlockable check valve is principally designed like the above-described check valve but includes a piston rod which is integrally connected with the piston-type control element and is displaceable in axially guided sealing relationship in a housing set having an outlet space sealed in a fluid-tight manner from the control space. Thus, the control element is not pressure-equalized in a neutral position of the check valve such as when the prop has been set, that is, the cylinder spaces of the control space on both sides of the piston-type control element are subjected to different pressure medium pressures.
The advantage of such an unlockable check valve is that the pressure peaks originating from an expansion blow, cannot act, through the outlet space, on the annular face of the piston-type control element with which the piston rod is associated. Consequently, expansion blows do not lead to a displacement of the control element against a control pressure. The pressure of the pressure fluid remains, therefore, about the same in the outlet chamber and can diminish slowly through the return line to the return flow channel connected here.
A disadvantage of this known design is that the fluid pressure can unilaterally act on the control element due to a back-pressure in the return flow line. It is thus possible, for example, that the dynamic back-pressure pressure will displace the control element, and thereby the piston rod, in the direction of the shutoff device to move the shutoff device into the open position. This may cause an accidental relief of the prop space which is pressurized by the pressure or working fluid, so that the prop gives way accidentally. In practice it happens now and again that hydraulic mine props are set with a low fluid pressure to protect the roof. In the case of accidental relief in props set at such low pressure, the roof will no longer underpin, which can have serious consequences.